in mahabharat, draupadi was NOT the reason for tht war. Dhritrashtra refused to give pandavas their indraprastha back that's why they HAD to fight the war. draupadi's insult was ONE of the reasons BUT shri Krishna rightly said that this can't be the ONLY reason for the war. her insult was not bigger than the blood loss which happened.
and in case of Prithviraj, well he was not the ONLY king to have many wives. All of them had several wives. Does anyone know abt the socio-political scenario at tht time? how can we judge whether or not Sanyogita was responsible? if Jaichand had helped Prithvi then they cud have easily defeated ghori. nobody sees this point bas they keep blaming sanyogita. again she might not be the ONLY factor for his defeat i'm sure.